The overall objective of the research is to investigate the molecular and genetic mechanisms of cellular repair of damaged nuclear and mitochondrial DNA and the effect of cellular repair mechanisms on genetic recombination and on the induction of nuclear and mitochondrial gene mutations. These problems are being studied in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae, where both mutants with repair deficiencies exist, and a well-defined mutational system is available. Repair of ultraviolet-light (UV) induced damage in nuclear and mitochondrial DNA is being measured by determining the susceptibility of dimer-containing DNA to the production of single strand breaks after treatment with T4 UV endonuclease. Repair of DNA damaged by chemical mutagens and carcinogens will be measured by determining the susceptibility of such damaged DNA to bacterial and other enzymes known to recognize specifically altered DNA. Reversion frequencies obtained in repair-proficient and repair-deficient mutants indicate what influence repair capacity has on the induction of mutations of nuclear and mitochondrial genes by physical agents such as ionizing radiation and UV light and by numerous chemical mutagens and carcinogens. Mutants of yeast sensitive to methyl methane-sulfonate (MMS) have been isolated in our laboratory and these mutants will be characterized by determining the effect of the genes conferring MMS-sensitivity on mitotic recombination, sporulation, meiotic recombination, induced mutation, repair capacity, and DNA polymerase activity. BIBLIOGRAPHIC REFERENCES: Prakash, Louise (1975) The effect of genes controlling radiation sensitivity on chemical mutagenesis in yeast. Molecular Mechanisms for Repair of DNA. Part A. pp. 393-395 (ed. P.C. Hanawalt and R.B. Setlow). Prakash, Louise (1976) Effect of genes controlling radiation sensitivity on chemically-induced mutations in Saccharomyces cerevisiae. Genetics, in press.